Is this going to be a surprise? Because for a lot of people guns are the type of personal thing that sometimes don’t work out well as a surprise. Please consider this to supercede my previous advice, but I think your best bet might be an invite for a joint trip to the gun store so he can pick out what he wants. I would dig it if an SO ever does something like that for me.
The only possible downside to that is that stock may be low depending on what part of the country you’re in.